The present invention relates to the formation of color in a self-coloring curable composition and to a method for forming highly colored polymeric bodies using such compositions.
The conventional method of obtaining strongly colored films by a photopolymerization process is by adding a colored dye to a photocurable or photopolymerizable composition containing one or more monomers and a photoinitiator. As long as the added dye does not absorb in the region at which the photoinitiator absorbs, the rate of polymerization will, in principle, not be affected by its presence. However, it is far more typical for the absorption spectra of the dye and the photoinitiator to seriously overlap and this slows the polymerization rate to an unacceptably low level.
In some cases spectral overlap can be avoided by the proper choice of the photoinitiator. A photoinitiator can be chosen that absorbs at longer wavelengths than the dye or at regions where the dye absorption has a minimum. Unfortunately, photoinitiators that absorb above 600 nm generally are not efficient. This means that, in cases where the added dye or dyes absorb strongly below 600 nm, photopolymerization at high speed is not feasible. A black ink, where carbon black or metal oxides are added as pigment is essentially uncurable.
Accordingly, there is a need for an efficient method for producing highly colored polymeric bodies such as thin polymeric films at high speed.